The present invention relates to a non-sintered nickel electrode used in particular as a positive electrode in a secondary electrochemical cell having an alkaline electrolyte, such as nickel metal hydride, nickel cadmium, nickel iron, nickel zinc, or nickel hydrogen storage batteries. The invention also relates to a cell containing the electrode, and to a method of preparing the electrode.
A non-sintered nickel electrode is made up of a two-dimensional support such as a continuous or perforated foil, an expanded metal, a grid, or a cloth, or indeed a three-dimensional support such as a foam or a felt, the support serving as a current collector. A paste containing the active material which is constituted by a nickel-based hydroxide and a binder, usually associated with a conductive material, is coated on the collector. Nickel hydroxide is a poorly conductive compound which requires conductive material to be included in the electrode to enable electricity to percolate well. As a general rule, the conductive material is a cobalt compound such as metallic cobalt Co, cobalt hydroxide Co(OH)2, and/or cobalt oxide CoO.
The first time the storage battery is charged, these compounds are oxidized into cobalt oxyhydroxide CoOOH in which the cobalt is taken to a degree of oxidation greater than or equal to +3. Cobalt oxyhydroxide is stable in the normal operating range of the nickel positive electrode and it is insoluble in the alkaline electrolyte. It enables electricity to percolate in the electrode.
For example, in order to accelerate the formation of cobalt oxyhydroxide CoOOH, document U.S. Pat. No. 5,405,714 proposes using metallic cobalt powder Co in the electrode together with nickel oxyhydroxide powder NiOOH which is the active material, at a concentration that is less than 60% by weight of the cobalt. The storage battery is left at rest until the potential of the positive electrode reaches that of the Co/Co(OH)2 couple, after which it is charged and discharged. The particles of cobalt Co are covered in a layer of cobalt oxyhydroxide CoOOH while the nickel oxyhydroxide NiOOH is reduced to the hydroxide state Ni(OH)2.
On initial charging, the oxidation of the cobalt compounds corresponds to equal quantities of electricity on the positive and negative electrodes. In addition, during subsequent discharges, the positive electrode is not fully discharged (oxidation degree 2) but is discharged only to the nickel being oxidized to degree 2.2. As a result, the non-discharged capacity or xe2x80x9cprechargexe2x80x9d of the negative electrode increases on each cycle, thereby progressively decreasing the effective capacity of said electrode and contributing to shortening the lifetime of the storage battery.
When stored in a fully discharged state, an alkaline storage battery possessing a non-sintered nickel positive electrode sees its voltage decrease over time. When the duration of storage exceeds a few months, its voltage tends towards 0 V. Under such conditions, cobalt oxy-hydroxide CoOOH reduces slowly. The cobalt is taken initially to oxidation degree +2.66 in Co3O4, and then it reaches oxidation degree +2 in Co(OH)2.
Unfortunately, cobalt hydroxide Co(OH)2 is a compound that is highly soluble in the electrolyte. After being stored for a period of several months, a loss of conductivity is observed due to part of the percolation network in the non-sintered electrode dissolving. This gives rise to an irreversible loss of capacity.
Document EP-0 789 408 proposes using nickel hydroxide powder having grains coated in a cobalt compound containing 0.1% to 10% by weight of sodium. Documents U.S. Pat. No. 5,672,447 and EP-0 798 801 propose covering a nickel hydroxide powder in a disordered cobalt compound of valency greater than +2. Such coatings are likewise not stable during storage at low potential.
In order to remedy that problem, European patent application EP-0 866 510 proposes an electrode containing nickel hydroxide as the main component with a conductive material constituted by an oxide of lithium and cobalt represented by the formula LixCoO2, where x lies in the range 0.2 to 0.9. The active material of the paste can also contain a mixture of nickel hydroxide powder and of nickel hydroxide powder in which the surface of the particles is coated in a layer of lithium and cobalt oxide, with lithium and cobalt oxide being added thereto as the conductive material. During storage of the cell, the observed irreversible loss of capacity is still too high.
An object of the present invention is to propose a non-sintered nickel electrode in which irreversible loss of capacity during storage in the discharged state is smaller than that of presently known electrodes.
Another object of the invention is to provide a nickel storage battery whose precharge is reduced by using a novel positive electrode.
The present invention provides a non-sintered nickel electrode for a secondary cell having an alkaline electrolyte, the electrode comprising a current collector and a paste comprising an active material in powder form based on nickel hydroxide, a conductive material containing lithium and cobalt, and a binder, the electrode being characterized in that said active material is constituted by particles of a hydroxide containing a majority of nickel that is at least partially oxidized into xcex2 structure oxyhydroxide, said particles being at least partially coated in said conductive material which is a lithiated oxide of nickel and cobalt.
In order to ensure that the positive electrode has an optimum usage ratio, the conductivity of said conductive material is greater than 10xe2x88x922 Siemens.cmxe2x88x921 after at least one charge/discharge cycle, referred to as xe2x80x9celectrochemical formingxe2x80x9d.
In a preferred embodiment, said lithiated oxide of nickel and cobalt has the formula LixNiyCO1-yO2 where 0.1xe2x89xa6xxe2x89xa61 and 0xe2x89xa6yxe2x89xa60.9, and preferably 0.02xe2x89xa6yxe2x89xa60.9.
The degree of oxidation of the cobalt in said lithiated oxide is not less than 3, and preferably equal to or greater than 3.2.
In another embodiment, the lithiated oxide contains sodium. Preferably, the lithiated oxide has the formula LixNazNiyCo1-yO2 where x+z lies in the range 0.1 to 1 and z lies in the range 0 to 0.5, i.e. 0.1xe2x89xa6x+zxe2x89xa61 and 0xe2x89xa6zxe2x89xa60.5.
Preferably, the quantity of lithiated oxide lies in the range 3% to 9% by weight relative to said active material.
In a variant, said paste also contains the powder form of the lithiated oxide of nickel and cobalt.
The conductive material occupies at least part of the micropores in the surface of the nickel-based hydroxide particle. This is the microporosity that is accessible to the electrolyte and that contributes to the electrochemically active surface of the hydroxide.
Without modifying the invention, the nickel-based hydroxide particles can be of various shapes, going from a more or less spherical shape to an irregular shape.
In a preferred implementation of the invention, 5% to 35% by weight of said nickel hydroxide Ni(OH)2 is oxidized into a xcex2 structure oxyhydroxide NiOOH, and preferably 5% to 20% by weight, and more preferably still 10% to 20%.
It is important that the xcex3-NiOOH oxyhydroxide does not form since the xcex3 phase has lattice parameters that are larger than those of the xcex2 phase. This characteristic of the xcex3 phase gives rise to breaks and to partial destruction in the coating which harms the performance of the electrode, and in particular harms its ability to conserve storage capacity.
It should naturally be understood that the term xe2x80x9celectrochemically active material containing nickel hydroxidexe2x80x9d as used in the present application can mean nickel hydroxide, a hydroxide that contains mainly nickel, and also a nickel hydroxide containing at least one syncrystallized hydroxide of an element selected from zinc (Zn), cadmium (Cd), magnesium (Mg), and aluminum (Al), and at least one syncrystallized hydroxide of an element selected from cobalt (Co), manganese (Mn), aluminum (Al), yttrium (Y), calcium (Ca), strontium (Sr), zirconium (Zr), and copper (Cu).
A syncrystallized hydroxide contained in nickel hydroxide is a hydroxide that forms a solid solution with nickel hydroxide, i.e. that occupies a continuously variable fraction of the atomic sites defined by the crystal lattice of the nickel hydroxide.
Said current collector is advantageously a nickel foam having porosity of not less than 90%.
The paste contains a binder which ensures that the active layer adheres to the collector.
In a first variant, said binder is a mixture of a crystalline polymer and an elastomer. Preferably, the proportion of said crystalline polymer lies in the range 40% to 75% by weight of said binder and the proportion of said elastomer lies in the range 25% to 60% by weight of said binder.
The crystalline polymer can be selected from a fluorine-containing polymer such as a copolymer comprising fluorinated ethylene propylene (FEP), poly-propylhexafluoride (PPHF) or polytetrafluoroethylene (PTFE), and a polyolefin such as polyethylene (PE).
The elastomer can be selected from a copolymer of styrene, ethylene, butadiene, and styrene (SEBS), a terpolymer of styrene, butadiene, and vinylpyridine (SBVR), and a copolymer of styrene and butadiene (SBR).
In a second variant, said binder comprises a first component selected from a fluorine-containing polymer such as polytetrafluoroethylene (PTFE), and at least one second component selected from a cellulose compound such as carboxymethylcellulose (CMC), hydroxypropylmethyl-cellulose (HPMC), hydroxyethylcellulose (HEC), and hydroxypropylcellulose (HPC), and a fluorine-containing compound such as polyvinylidene fluoride (PVDF), and an elastomer selected as a copolymer of styrene and butadiene (SBR).
In order to make the electrode easier to manufacture, the paste can also contain a thickener such as a cellulose compound selected from as carboxymethylcellulose (CMC), hydroxypropylmethylcellulose (HPMC), hydroxypropylcellulose (HPC), and hydroxyethylcellulose (HEC).
The paste can also contain at least one other compound selected from zinc compounds such as ZnO or Zn(OH)2, yttrium compounds such as Y2O3 or Y(OH)3, and calcium compounds such as CaO, Ca (OH)2, or CaF2. This compound is preferably added in powder form.
The present invention also provides a method of manufacturing an electrode of the present invention as described above. The method comprises the following steps:
preparing a solution containing an oxidizing agent;
immersing particles of nickel hydroxide coated in said conductive material in said solution;
leaving said particles in contact with said solution;
separating said oxidized particles from said solution; and
washing and drying said oxidized particles.
Said oxidizing agent is preferably selected from sodium hypochlorite and calcium hypochlorite. The oxidizing solution is preferably an aqueous solution of sodium hypochlorite or of calcium hypochlorite. The quantity of oxidizing agent used lies in the range once to three times the stoichiometric quantity required for oxidizing said nickel hydroxide.
The temperature of the solution is 20xc2x0 C., but it can be raised to up to 90xc2x0 C. without modifying the characteristics of the resulting products. The temperature of the solution is preferably no more than 40xc2x0 C.
The duration of contact between the nickel hydroxide particles and the solution lies in the range one hour to three hours, which is sufficient, but the duration of contact can be lengthened without impediment.
The drying temperature lies in the range 40xc2x0 C. to 100xc2x0 C. for a duration lying in the range 12 hours to 48 hours.
The present invention also provides a secondary electrochemical cell of the nickel-metal hydride type comprising:
a positive electrode of the present invention as described above;
a separator;
a negative electrode whose electrochemically active material is an intermetallic compound capable of forming a hydride when charged, the total quantity of electro-chemically active material of said negative electrode exceeding the total quantity of electrochemically active material of said positive electrode in such a manner that the total negative capacity exceeds the total positive capacity by a quantity referred to as xe2x80x9cover-capacityxe2x80x9d, a portion of said over-capacity, referred to as xe2x80x9cprechargexe2x80x9d, being partially in the charged state once said positive electrode has been fully discharged, said precharge representing less than 12% of said negative capacity; and
an alkaline aqueous electrolyte.
Other characteristics and advantages of the present invention will appear on reading the following description of embodiments given by way of non-limiting illustration.